The present invention relates to an error correction system for a measuring instrument of the type comprising a measuring scale and a scanning unit positioned to scan the scale, wherein the error correction system includes an error correction profile, at least one adjusting element for adjusting the position of the correction profile transversely to the measuring direction, a transfer element coupled between the scanning unit and an object to be measured in order to vary the separation therebetween in response to the profile, and an error correction scale positioned adjacent to the adjusting element.
German Patent DE-PS No. 31 36 981 discloses a length measuring system in which an error correction profile is provided for error correction. This profile is adjustable by means of eccentrics transversely with respect to the measuring direction.
The publication of the firm of Dr. Johannes Heidenhain GmbH entitled "Inkrementales LangenmeBsystem LS 107" (October, 1982) discloses a length measuring system in which an error correction profile is adjustable by means of eccentrics transversely to the measuring direction for error correction. In order to facilitate adjustment of the error correction profile the eccentrics each define an index mark which cooperates with a fixed scale on the housing of the measuring system to indicate the position of the individual eccentrics.
In general, the errors to be corrected by such an error correction system are made up of two general classes of errors: (1) graduation errors or division errors of the measuring graduation of the measuring system, and (2) machine errors such as guidance errors of the machine parts, the position of which is to be calibrated. Ordinarily, such a measuring system is calibrated prior to delivery, for example by means of a laser interferometer, and graduation errors of the measuring graduation are corrected by adjusting the error correction profile by means of the eccentrics. As a general matter, after the correction of these division or graduation errors at least a portion of the adjusting marks fixedly positioned on the eccentrics deviate from the zero line of the fixed scale. After such a measuring system has been attached to a processing or measuring machine in general it will be necessary to correct the system again for potential guidance errors of the machine parts to be measured. Likewise, this is done by adjusting the position of the error correction profile by means of the eccentrics. For example, a previously measured error record can be used to set the eccentrics in order to correct for guidance errors. The entry of corrections for such guidance errors is complicated however by the fact that not all of the index marks of the eccentrics start out at the zero line of the scale. As explained above this is due to the fact that the fixed index marks have at least in part been moved from the zero line of the fixed scale in order to adjust for graduation errors. For this reason, a reproduceable and an accurate setting of the error correction profile in correspondence with the guidance error correction is difficult and not always free of error, even with later readjustments.